Zero to the power of zero, denoted by 00, is a mathematical expression that is either defined as 1 or left undefined, depending on context. In algebra and combinatorics, one typically defines 00 = 1. In mathematical analysis, the expression is sometimes left undefined. Computer programming languages and software also have differing ways of handling this expression.
Discrete exponents
Many widely used formulas involving natural-number exponents require 00 to be defined as 1. For example, the following three interpretations of b0 make just as much sense for b = 0 as they do for positive integers b:
- The interpretation of b0 as an empty product assigns it the value 1.
- The combinatorial interpretation of b0 is the number of 0-tuples of elements from a b-element set; there is exactly one 0-tuple.
- The set-theoretic interpretation of b0 is the number of functions from the empty set to a b-element set; there is exactly one such function, namely, the empty function.
All three of these specialize to give 00 = 1.
Polynomials and power series
When evaluating polynomials, it is convenient to define 00 as 1. A (real) polynomial is an expression of the form a0x0 + ⋅⋅⋅ + anxn, where x is an indeterminate, and the coefficients ai are real numbers. Polynomials are added termwise, and multiplied by applying the distributive law and the usual rules for exponents. With these operations, polynomials form a ring R[x]. The multiplicative identity of R[x] is the polynomial x0; that is, x0 times any polynomial p(x) is just p(x). Also, polynomials can be evaluated by specializing x to a real number. More precisely, for any given real number r, there is a unique unital R-algebra homomorphism evr : R[x] → R such that evr(x) = r. Because evr is unital, evr(x0) = 1. That is, r0 = 1 for each real number r, including 0. The same argument applies with R replaced by any ring.
Defining 00 = 1 is necessary for many polynomial identities. For example, the binomial theorem (1 + x)n = Σn
k=0 (n
k) xk holds for x = 0 only if 00 = 1.
Similarly, rings of power series require x0 to be defined as 1 for all specializations of x. For example, identities like 1/1−x = Σ∞
n=0 xn and ex = Σ∞
n=0 xn/n! hold for x = 0 only if 00 = 1.
In order for the polynomial x0 to define a continuous function R → R, one must define 00 = 1.
In calculus, the power rule d/dxxn = nxn−1 is valid for n = 1 at x = 0 only if 00 = 1.
Continuous exponents
Limits involving algebraic operations can often be evaluated by replacing subexpressions by their limits; if the resulting expression does not determine the original limit, the expression is known as an indeterminate form. The expression 00 is an indeterminate form: Given real-valued functions f(t) and g(t) approaching 0 (as t approaches a real number or ±∞) with f(t) > 0, the limit of f(t)g(t) can be any non-negative real number or +∞, or it can diverge, depending on f and g. For example, each limit below involves a function f(t)g(t) with f(t), g(t) → 0 as t → 0+ (a one-sided limit), but their values are different:
Thus, the two-variable function xy, though continuous on the set {(x, y) : x > 0}, cannot be extended to a continuous function on {(x, y) : x > 0} ∪ {(0, 0)}, no matter how one chooses to define 00.
On the other hand, if f and g are analytic functions on an open neighborhood of a number c, then f(t)g(t) → 1 as t approaches c from any side on which f is positive. This and more general results can be obtained by studying the limiting behavior of the function ln(f(t)g(t)) = g(t) ln f(t).
Complex exponents
In the complex domain, the function zw may be defined for nonzero z by choosing a branch of log z and defining zw as ew log z. This does not define 0w since there is no branch of log z defined at z = 0, let alone in a neighborhood of 0.
History
As a value
In 1752, Euler in Introductio in analysin infinitorum wrote that a0 = 1 and explicitly mentioned that 00 = 1. An annotation attributed to Mascheroni in a 1787 edition of Euler's book Institutiones calculi differentialis offered the "justification"
As a limiting form
Euler, when setting 00 = 1, mentioned that consequently the values of the function 0x take a "huge jump", from ∞ for x < 0, to 1 at x = 0, to 0 for x > 0. In 1814, Pfaff used a squeeze theorem argument to prove that xx → 1 as x → 0+.
On the other hand, in 1821 Cauchy explained why the limit of xy as positive numbers x and y approach 0 while being constrained by some fixed relation could be made to assume any value between 0 and ∞ by choosing the relation appropriately. He deduced that the limit of the full two-variable function xy without a specified constraint is "indeterminate". With this justification, he listed 00 along with expressions like 0/0 in a table of indeterminate forms.
Apparently unaware of Cauchy's work, Möbius in 1834, building on Pfaff's argument, claimed incorrectly that f(x)g(x) → 1 whenever f(x),g(x) → 0 as x approaches a number c (presumably f is assumed positive away from c). Möbius reduced to the case c = 0, but then made the mistake of assuming that each of f and g could be expressed in the form Pxn for some continuous function P not vanishing at 0 and some nonnegative integer n, which is true for analytic functions, but not in general. An anonymous commentator pointed out the unjustified step; then another commentator who signed his name simply as "S" provided the explicit counterexamples (e−1/x)x → e−1 and (e−1/x)2x → e−2 as x → 0+ and expressed the situation by writing that "00 can have many different values".
Current situation
- Some authors define 00 as 1 because it simplifies many theorem statements. According to Benson (1999), "The choice whether to define 00 is based on convenience, not on correctness. If we refrain from defining 00, then certain assertions become unnecessarily awkward. ... The consensus is to use the definition 00 = 1, although there are textbooks that refrain from defining 00." Knuth (1992) contends more strongly that 00 "has to be 1"; he draws a distinction between the value 00, which should equal 1, and the limiting form 00 (an abbreviation for a limit of f(t)g(t) where f(t), g(t) → 0), which is an indeterminate form: "Both Cauchy and Libri were right, but Libri and his defenders did not understand why truth was on their side."
- Other authors leave 00 undefined because 00 is an indeterminate form: f(t), g(t) → 0 does not imply f(t)g(t) → 1.
There do not seem to be any authors assigning 00 a specific value other than 1.
Treatment on computers
IEEE floating-point standard
The IEEE 754-2008 floating-point standard is used in the design of most floating-point libraries. It recommends a number of operations for computing a power:
pown(whose exponent is an integer) treats 00 as 1; see § Discrete exponents.pow(whose intent is to return a non-NaN result when the exponent is an integer, likepown) treats 00 as 1.powrtreats 00 as NaN (Not-a-Number) due to the indeterminate form; see § Continuous exponents.
The pow variant is inspired by the pow function from C99, mainly for compatibility. It is useful mostly for languages with a single power function. The pown and powr
variants have been introduced due to conflicting usage of the power
functions and the different points of view (as stated above).
Programming languages
The C and C++ standards do not specify the result of 00 (a domain error may occur). But for C, as of C99, if the normative annex F is supported, the result for real floating-point types is required to be 1 because there are significant applications for which this value is more useful than NaN (for instance, with discrete exponents); the result on complex types is not specified, even if the informative annex G is supported. The Java standard, the .NET Framework method System.Math.Pow, Julia, and Python also treat 00 as 1. Some languages document that their exponentiation operation corresponds to the pow function from the C mathematical library; this is the case with Lua and Perl's ** operator (where it is explicitly mentioned that the result of 0**0 is platform-dependent).
